I agree that it sounds like a bit of a double standard. but I think you have an excellent base for discussion - when she got her boyfriend, did she plan on leaving you?
If she didn't, then get her to talk a little about why not. Get her to talk in as much detail as she can. Then try to identify what, if anything, is substantially different in your case.
If she DID plan on leaving you and didn't for some reason, then you can ask her why she didn't....
"Listen, or your tongue will make you deaf." - Native American Proverb